Jarv/common refactor

# Summary
* The entire application is installed under a single directory, for
now the default is /edx but it's easy to change.
* edxapp and forums now run in supervisor

```
/edx/bin/supervisorctl -c /edx/etc/supervisord.conf 
edxapp:cms                       RUNNING    pid 16884, uptime 0:04:16
edxapp:lms                       RUNNING    pid 16883, uptime 0:04:16
edxapp_worker:cms_default_4      RUNNING    pid 16902, uptime 0:04:16
edxapp_worker:cms_high_1         RUNNING    pid 16891, uptime 0:04:16
edxapp_worker:cms_low_3          RUNNING    pid 16886, uptime 0:04:16
edxapp_worker:lms_default_3      RUNNING    pid 16887, uptime 0:04:16
edxapp_worker:lms_high_4         RUNNING    pid 16907, uptime 0:04:16
edxapp_worker:lms_low_1          RUNNING    pid 16895, uptime 0:04:16
forum                            RUNNING    pid 16885, uptime 0:04:16
```

* Strips the common role of tasks that are no longer necessary
(touching log files, changing permissions, etc)
* Takes the virtualenv creation out of the common role as well as
other jobs that are now in edxapp
* Makes the edxapp role conform to variable naming conventions
* gunicorn and nginx run as www-data but the rbenv, virtualenv,
staticfiles dir and code repo is now owned by the "edxapp" user.  This
is important for security and should be the convention moving forward
for other roles.
* This pull aggressively removes extra tags, for now I am assuming we don't want role tags on every task in a role.
* Application files and application configuration - `/edx/app/<role>`
* Application data - `/edx/var/<role>`
* Application logging - `/edx/var/log/<role>`
* Application python virtual environments - `/edx/app/<role>/venvs/<name>`
* Application ruby environment - `/edx/app/<role>/{.rbenv,.gem}`
* `/edx/bin` has symlinks as a convenience for running executables in the app/ dir, for now this directory is flat without any subdirectories
* `/edx/etc` has symlinks to configuration directories for convenience, for now this directory is flat without any subdirectories
* nginx configuration `{sites_enabled, sites_available}` has been relocated to `/edx/app/nginx`


# TODOs

These TODOs have been put on the backlog

* the user that the forum runs as (www-data) needs write access to its .gem/ directory, for this reason its rbenv is owned by www-data which should be fixed
* applications keep their configuration in the /app directory, this is necessary for now until we go and update repos not to use relative paths to load the json files.
* xqueue, xserver, ora are still running under upstart, they should be moved to supervisor after this pull is merged and checked out
* need to discuss whether other services, nginx, rabbit, mongo, etc. should be moved to supervisor